Porous polymeric membranes are well known in the art and are used widely for filtration and purification processes, such as filtration of waste water, preparation of ultra pure water and in medical, pharmaceutical or food applications, including removal of microorganisms, dialyses and protein filtration. Porous polymeric membranes are used to separate components of liquid mixtures by membrane distillation and as contactors to facilitate dissolution of gases in liquids or to remove gases from liquids, as membrane bioreactors, and in numerous other applications where they serve as a generic phase separator, for example, as a battery separator. Composite polymeric membranes that consist of a dense separation layer superimposed on porous support are used in gas separations such as natural gas treatment, gas dehydration, and hydrogen recovery from petrochemical and refinery streams. These composite membranes can be further utilized for removal of dissolved gases from liquids and for dehydration of liquids. While these membranes have found broad utility for a variety of purposes, they suffer from several disadvantages: broad and non uniform pore size distribution, limited chemical, solvent and thermal resistance, and surface characteristics, in particular when non-wetting, oleophobic properties are required. Porous polyolefin membranes, such as polypropylene and polyethylene membranes, are utilized for membrane distillation and as membrane contactors to promote dissolution or removal of gases from liquids. However, these membranes frequently wet out by the liquid media which leads to reduction in mass transfer and an inferior performance. Porous membranes with improved surface properties are thus required for continuous stable operation of membrane contactors. Furthermore, commercial porous polymeric membranes exhibit limited solvent resistance that limits the scope of their application. Low cost porous polymeric membranes with tailored surface characteristics, uniform pore size distribution, improved thermal stability and solvent resistance are thus still needed.
Poly(aryl ether ketone)s represent a class of semi-crystalline engineering thermoplastics with outstanding thermal properties and chemical resistance. One of the representative polymers in this class is poly(ether ether ketone), PEEK, which has a reported continuous service temperature of approximately 250° C. PAEK polymers are virtually insoluble in all common solvents at room temperature. These properties make PAEK attractive materials for porous membrane preparation. However, application of PAEK polymers to fabrication of membranes has been limited owing to their intractability, which prevents the use of conventional solvent-based methods of membrane casting. PAEK polymers can be chemically modified to impart solubility, for example, by sulfonation. However, articles formed from such functionalized PAEK polymers lose many of the desired properties. Bulk modification leads to a disruption in polymer chain crystallization and articles subsequently formed from such functionalized polymers loose solvent resistant properties.
A number of methods to prepare porous PAEK membranes have been disclosed in the art. It is known to prepare porous PEEK membranes from solutions of strong acids, such as concentrated sulfuric acid. However, PEEK can undergo sulfonation in the concentrated sulfuric acid media and thus can loose some of its desirable sought after properties. U.S. Pat. No. 6,017,455 discloses preparation of non-sulfonated porous PEEK membranes from concentrated sulfuric acid solvents sufficiently diluted by water to prevent sulfonation. The membranes are formed by casting PEEK solution to form a film followed by coagulation in a concentrated sulfuric acid. This membrane preparation process is complicated and produces large amounts of waste acid.
U.S. Pat. No. 5,997,741 discloses preparation of porous PEEK membranes by forming a solution of PEEK polymer in a concentrated sulfuric acid at the temperature of 15° C. or lower to prevent sulfonation. The solution is processed and cast at a sub ambient temperature, followed by coagulation in water or in a concentrated sulfuric acid. Only dilute PEEK solutions can be formed in the concentrated sulfuric acid which adversely affects film forming characteristics, the mechanical characteristics, and the pore morphology of the thus formed porous PEEK membranes.
U.S. Pat. Nos. 4,992,485 and 5,089,192 disclose preparation of PEEK membranes from non-sulfonating acid solvents that include methane sulfonic acid and trifluoromethane sulfonic acid. European Patent Specification EP 0737506 discloses preparation of improved polymeric membranes based on PEEK admixtures with polyethylene terephthalate. The membranes are formed by the solution casting process from a methane sulfuric acid/sulfuric acid solvent mixture.
The acid based solvent systems for manufacturing of porous PEEK membranes disclosed in the art are highly corrosive, frequently toxic and generate substantial environmental and disposal problems. For these and other reasons, the acid based casting processes have found limited commercial use.
An alternative to the acid based solvent system for PEEK membrane preparation involves the use of high boiling point solvents and plasticizers that dissolve PEEK polymer at elevated temperatures. U.S. Pat. Nos. 4,957,817 and 5,064,580, both issued to Dow Chemical Co., disclose preparation of porous PEEK articles from its admixture with organic polar solvents having a boiling point in the range of 191° C. to 380° C., such as benzophenone and 1-chloronaphthalene, and organic plasticizers capable of dissolving at least 10 weight percent of PEEK, respectively. The final porous article is formed by removing the organic polar solvents and/or plasticizers by dissolution into a low boiling temperature solvent. U.S. Pat. No. 5,200,078 discloses preparation of microporous PEEK membranes from its mixtures with plasticizers wherein the membrane undergoes a drawing step prior to or after the plasticizer is removed by leaching. U.S. Pat. No. 5,227,101 issued to Dow Chemical Co. discloses preparation of microporous membranes from poly(aryl ether ketone) type polymer by forming a mixture of PEEK type polymer, a low melting point crystallizable polymer, and a plasticizer, heating the resulting mixture, extruding or casting the mixture into a membrane, quenching or coagulating the membrane and leaching the pore forming components. U.S. Pat. No. 5,205,968, issued to Dow Chemical Co., discloses preparation of microporous membranes from a blend containing a poly(aryl ether ketone) type polymer, an amorphous polymer and a solvent.
M. F. Sonnenschein in the article entitled “Hollow fiber microfiltration membranes from poly(ether ether ketone)”, published in the Journal of Applied Polymer Science, Volume 72, pages 175-181, 1999, describes preparation of PEEK hollow fiber membranes by thermal phase inversion process. The use of a leachable additive polymer, such as polysulfone, is proposed to enhance membrane performance. Preparation of porous PEEK membranes by coextrusion of PEEK with polysulfone polymers followed by the dissolution of the polysulfone polymer from the interpenetrating network is disclosed in European Patent Application 409416 A2.
It is also known in the art to prepare porous PEEK membranes from its blends with a compatible poly(ether imide) polymer, PEI. Preparation of such membranes is described by R. S. Dubrow and M. F. Froix in U.S. Pat. No. 4,721,732 and by R. H. Mehta et al. in an article entitled “Microporous membranes based on poly(ether ether ketone) via thermally induced phase separation”, published in the Journal of Membrane Science, Volume 107, pages 93-106, 1995. The porous structure of these PEEK membranes is formed by leaching the poly(ether imide) component with an appropriate strong solvent such as dimethylformamide. However, as described by Mehta et al., the quantitative removal of PEI component by leaching is essentially impossible which in turn can lead to an inferior membrane performance.
Japan Kokai Tokkyo Koho 91273038 assigned to Sumitomo Electric Industries, Ltd., discloses preparation of porous PEEK membranes by an ion track etching method.
M. L. Bailey et al. in U.S. Pat. No. 5,651,931 describe a sintering process for the preparation of biocompatible filters, including PEEK filters. The filters are formed from a PEEK powder of a pre-selected average particle size by first pressing the powder into a “cake” followed by sintering in an oven or furnace. The process is limited to preparation of filters with a relatively large pore size and a broad pore size distribution and does not provide economic means of forming large membrane area fluid separation devices.
A process for preparation of porous PAEK articles that preserves the desirable thermal and chemical characteristics of PAEK polymers has been recently disclosed in U.S. Pat. No. 6,887,408.
Perfluoropolymers exhibit superior chemical durability and thus are sought after materials for preparation of membranes. Porous perfluoropolymer membranes are utilized for variety of filtration separation applications while non-porous amorphous perfluoropolymers are utilized in gas and vapor separation applications and gas transfer. Commercial porous perfluoropolymer membranes are typically available in a relatively large pore size (above 0.1 micrometer) and suffer from a broad and non uniform pore size distribution that can limit their use as separation membranes and as gas/liquid transfer membranes due to surface wet out. Stand alone porous perfluoropolymer membranes tend to compact under high cross membrane differential pressures due to a relatively low modulus and tensile strength of perfluoropolymers. An attempt to remedy these deficiencies was made by preparing composite perfluoropolymer membranes. U.S. Pat. No. 4,754,009 to Squire discloses a gas permeable material that contains passageways wherein the interior of the passageways is formed by solution coating of perfluoro-2,2-dimethyl-1,3-dioxole. U.S. Pat. No. 6,540,813 to J. K. Nelson et al. discloses preparation of composite membranes from perfluoropolymers by depositing a thin non-occlusive layer of perfluoropolymer on the exterior surface of a porous support. U.S. Pat. No. 5,876,604 to Nemser et al. discloses preparation of composite perfluoro-2,2-dimethyl-1,3-dioxole membranes that can be useful in gas transfer to and from liquids.
U.S. Pat. No. 5,051,114 to Nemser et al. discloses amorphous perfluoro-2,2-dimethyl-1,3-dioxole based polymers that can be used for gas separation and gas enrichment applications. U.S. Pat. No. 6,406,517 to D. L. Avery et al. discloses preparation of gas permeable membranes from blend of perfluoropolymers with non-fugitive, non-polymeric fluorinated adjuvant. U.S. Pat. No. 6,544,316 to R. W. Baker et al. discloses gas separation membranes with selective layer formed from fluorinated polymer resistant to plasticization by the organic components in the gas mixture.
The prior art composite perfluoropolymer membranes are formed by depositing a perfluoropolymer onto a porous support. The porous support is typically formed from a conventional polymeric material such as polysulfone or polyetherimide. These porous supports are not sufficiently chemically or thermally resistant and degrade in long term operation. The perfluoropolymer is not attached chemically to the porous support and thus the perfluoropolymer layer can delaminate and the membrane can otherwise develop defects due to the differential thermal expansion of coating and substrate materials and/or substrate swelling when subjected to contact with solvent media or condensable vapor.
Thus there are still remains a need in the art for composite porous perfluoropolymer membranes with a chemically resistant substrate and a uniform narrow pore size distribution with pore diameter below 100 nm.
Poly (aryl ether ketone)s are high performance engineering polymers that exhibit exceptional thermal and chemical characteristics and are thus highly sought after as porous substrates for composite membrane preparation. However, the properties that make PAEK polymers desirable also make preparation of porous membranes difficult. Chemical resistance of PAEK polymers makes the functional modification of the preformed porous article difficult and such functionalized porous PAEK articles are unknown. Preparation of porous PAEK membranes with perfluoro hydrocarbon surface layers is not known in the art.
A number of techniques have been used in the art to chemically modify the surface of dense PEEK films to affect surface characteristics such as friction, wettability, adsorption and adhesion, including cell adhesion. O. Noiset, et al., have modified the PEEK film surface using wet-chemistry technique by selectively reducing ketone groups to form hydroxyl groups and then covalently fixing hexamethylene diisocyanate by addition onto the hydroxyl function (Journal of Polymer Science, Part A, Vol. 35, pages 3779-3790, 1997). C. Henneuse-Boxus, et al., have modified PEEK film surfaces using photochemical routes (European polymer Journal, Vol. 37, pages 9-18, 2001). P. Laurens, et al., have modified PEEK surfaces with excimer laser radiation (Applied Surface Science, Vol. 138-139, pages 93-96, 1999). N. Franchina and T. McCarthy have modified semi-crystalline PEEK films with carbonyl-selective reagents to induce surface functionality (Macromolecules, Vol. 24, pages 3045-3049, 1991). The surface modified films were robust and unaffected by a variety of solvents. In U.S. Pat. No. 5,260,415 I. David disclosed a process for the crosslinking of polymers containing diaryl ketone groups by heating the polymer with alcohol and/or alkoxide to enhance chemical resistance.